Method of pickling metal contact surfaces



Patented June-l4, 1949 srs'mon or PICKLING METAL oou'rao'r SURFACES Edwin E. Bowel-man, Jr., Emporium, Pa., alslgnor to Sylvania Electric Products Inc.,' Emporium, 2a., a corporation of Massachusetts No Drawing. Application August 21, 1943,

. Serial No. 499.558

-1 8 Claims.

This invention refers to the treatment of metal surfaces and in particular to methods of producing electrical contact surfaces on metal or alloy pieces which may be covered with a tenacious oxide layer.

It is often desirable to obtain electrical contact surfaces on metal bodies which, due to heat treatment needed for obtaining certain physical or structural changes, are covered with a non-conducting, or poorly conducting oxide layer. This oxide layer may be diflicult to remove, as is true, e. g., in the case of certain stainless-steel alloys, such as those of the Ni-Cr-Fe class.

The practical diiliculties are increased if it is desired to keep the shape and dimensions of the metal pieces uniform and in agreement with given standard specifications within close tolerance limits.

A good electrical contact surface may be desired for various purposes. It may be used for attaching another metal piece to the surface by resistance welding, or the surface may itself form onehalf of an electric contact, as in a switch, a relay, or as the contact pins of radio tubes which make the outside circuit connections to other circuit elements of a device incorporating electron discharge tubes, as in a radio receiving set.

Merely as an example of a metal piece in which both a good resistance welding contact and a good temporary circuit connecting contact is required, let us consider the case of a lead-in connector sealed into the glass header of an electron discharge tube, e. g., as described in U. S. Patent No. 2,250,184 (issued to Carl F. Miller, July 22, 1941).

The part of the lead-in connector pin which serves as acontact member to be introduced into the tube socket must not only be freed from oxide, but must also be made chemically passive, in order to insure a good contact surface after months or even years of storage, and when exposed to handling or to a deteriorating atmosphere.

The other end of such a lead-in connector, pin, which projects into the evacuated volume enclosed in the envelope of an electron tube must be e1ec-' trically connected to the tube electrodes. This is usually accomplished by resistance welding. A good electrical contact surface is here required, and passivating of the cleaned metal surface is desirable, because production schedules do not always render it feasible to make the weldin connections to the tube elements immediately after cleaning.

v The outer ends, which serve as socket contact members are, however, much more sensitive, because, in addition to the requirement of passivity, they must conform to very close geometrical tolerances. It is thus easy to understand the importance of a process which provides the removal of a tenacious oxide layer; passivates the exposed metal surface and avoids variations in the geometrical dimensions of the metal pieces.

In some cases, the mere passivating of the cleaned metal surface is not enough to insure a permanent good electrical surface contact, particularly when the contact pieces are exposed to salt water or to the high humidity of tropical climates. Base metals will not stand up for a long time under these conditions without undergoing chemical reactions leading to poorly conductive surface layers, and it becomes necessary to employ a surface coating of a more noble metal, such as silver or gold, on' the parts of the metalpieces exposed to the open air at high temperatures.

It is an object of the invention to provide means for removing tenacious oxide coatings from the surface of ferrous and other alloys.

, According to another object of the invention,

2 means are provided for removing tenacious oxides happens that a cleaning tool, in addition to scrap- 'ing and cutting the oxide layer on the metal surface, has the effect of imbeddlng parts of the oxide and small particles of its own material into the metal surface which it is supposedto clean.

A well-known chemical method employs boiling in an alkaline KMnOr solution. This method exposes the operator to the disagreeable and harm- In many cases, it is therefore necessary to supplement the anodic electrocleaning by a mechanical grinding and polishing, which makes it expensive, and practically results in a too large spread of. the geometrical dimensions of the finished piece far beyond the tolerance limits.

Most of these methods disregard entirely the passivity requirements of obtaining permanent good electrical contact surfaces.

Accordingly, it is an object of the invention to provide a means for removing oxide from steel surfaces which (a) eliminates the development of obnoxious fumes,

(b) eliminates the development of pits and other surface irregularities, and

() produces a high degree of passivity of the exposed clean metal surface.

One feature of the invention relates to a method of removing tenacious oxide coatings from pinshaped metal bodies which decreases the outside diameter of the pin-shaped body by less than .Qdi". 1

A feature of the invention relates to a method of cleaning stainless steel surfaces, which results in uniform resistance welding. properties.

A further feature relates to a method of preparing a uniformly conducting metal surface which can readily be plated with another metal.

A still further object of the invention relates to a process which reduces the electrode potential of a stainless steel alloy to less than .15 volts measured against a platinum electrode in a concentrated KCl solution.

The cleaning and passivating process accord! ing to the invention is carried out in three steps. While the passivating consists in simple immersion in commercially concentrated nitric acid, the preceding cleaning process is split into two steps which permit simple control by means of temperature and timing schedules. According to thetenacious nature and the hardness of the oxide coatings under consideration, their removal by chemical action is based on the principle of removing part of .the metal layer immediately below the oxide coating. The tenacious mechanical bond between oxide and metal surface is thus released, eliminating a frontal attack against the very hard oxide surface, which cannot be carried out without inflicting permanent localized injury to certain parts of the underlying metal layer, down to an undesirable depth as in wire brush cleaning or anodic electrolysis.

The process according to the invention uses the method of infiltration, whereby the pores of the oxide are used as passages for getting at the metal surface itself. Clogging up of these minute pores must be avoided, and hydrolysis of the etching solution must therefore be prevented. On the other hand, the infiltration must be uniformly distributed over the exposed surface, and the action of the solution penetrating through the pores must not be too violent, as such action would make the process hard to control.

Accordingly, the first or preliminary cleaning step applies a mild etching solution which activates the metal. A wetting agent is added. to

facilitate penetration. Violent action aswell as hydrolysis must be avoided in this step. The mild etching of the alloy surface undermines the bond between oxide and core metal. The principle purpose of this first step is, however, the activation of the iron at the metal surface. This conditions the surface so as to ease solution of the iron which is carried out in the second step.

The second cleaning step has for its main purpose a mechanical action, 1. e., the complete removal of the oxide layer, whose bond with the core metal was undermined in the first step. This is accomplished by a solution which dissolves the iron immediately below the oxide surface, which has been activated in the first step, and replaces it by a material which occupies a much larger volume than the iron which is bein dissolved. A convenient reaction of this type takes place in copper nitrate. which dissolves iron, and causes copper to accumulate at the surface. In this reaction, three copper atoms take the place of each group of two iron atoms going into solution in the bath, and, in addition, the copper deposits in a fluffy form, whereby the originally solidly packed iron is replaced by a corresponding copper .quantity occupying a very large volume. This reaction is known as copper flash." As it takes place between the surface layer of the core metal and the oxide coating, the process has a mild blasting action on the original oxide coating, which is thus completely shed, exposing the copper flashed core metal surface.

In order to remove the copper'flash, a copper solvent is added to the solution used in this second step. This copper solvent must act slowly enough so as to permit a temporary occurrence of the copper flash at the metal surface. A proper proportion of nitric acid in the solution leads to'the desired sequence of reactions. The control may be improved by the addition of phosphoric acid, which promotes the desired formation of slightly dissociated iron complexes.

After a certain period of time the reactions between the metal and the oxide coating surfaces have completely broken down the bond between oxide layer and core metal, resulting in a complete peeling of the oxide, and the exposure of the desired pure alloy surface, enriched in Cr and Ni.

But the metal surface, after this treatment is not yet sumciently deactivated. In order to insure a passive surface which is immune against re-oxidation (as caused by handling), the alloy parts are now treated in (commercially) concentrated nitric acid, until the desired degree of passivity is reached. Experimental tests have shown that it is possible to obtain a potential difference from platinum measured in a saturatedKCl solution, which is less than .15 volts.

The composition of the preferred solutions, which yield the desired results and a convenient temperature and timing schedule follow.

The material to be cleaned is placed in an activating solution containing FeCla plus an acid such as HCl. The use of a wetting agent is desirable. A preferred solution consists of 300 g. of FeCia, 3,00 0. c. of HCl (sp. gr. 1.18), and enough H20 to total 1 litre. The combination of HCl and FCC]: activates the steel alloy and provides etching of the alloy metals, thus starting the loosening of the oxide, and preparing for the flash-out of Cu which occurs later in the process. The time of immersion is dependent upon temperature, the time decreasing with an increase in temperature; e. g. the material may be immersed 1 minute at boiling temperature or 5 minutes at C.

After the above-specified solution is rinsed away, the alloy is immersed in a solution of copper nitrate and nitric acid to which an acid of the type HaPO4 or H2804 and water are added. Preferred specifications for this step of the cleaning rocess are: 25 g. of Cu(NO3)z.6H2O, 55 c. c. of 70% HaPO4, 200 c. c. of HNO: and H20 to make a total volume of 1 litre. Again the time of immersion is dependent upon temperature, e. g., 5 minutes at C. or 25 minutes at 50 C. The Cuin this bath tends to displace iron-on the surface of the steel alloy .to' be deoxidized. when immersed in the bath. the surface metal undergoes the change indicated by the equation:

HNOa dissolves the copper which has flashed out. Following immersion. the stems are rinsed in water and passivated in an HNO: solution in invention results in an electrode potential. of

the Ni-Cr-Fe alloy of less than .15 volt, compared with platinum.

As was mentioned hereinabove, it is sometimes desirable to keep the cleaned surface of a piece of metal or alloy in the condition of good surface conductivity for a long period of time and to protect it from oxidation due to subsequent exposure to high temperature or particularly deleterious atmospheric conditions. Under such circumstances, electroplating .of the alloy pieces with a noble metal (silver or gold) is advisable.

What is claimed is:

1. The method of removing a porous oxide coating from the surface of a ferrous base alloy which comprises lightly etching said alloy by treating its surface with a mild etching agent containing a wetting agent to effect an activation of the alloy surface and then immersing the alloy Number in a bath containing approximately 25 gms. of

copper nitrate and approximately 200 c. c.s of nitric acid per liter thereby first causing a replacement action in which iron on the surface of the alloy is replaced by copper an dissolved in the bath prior to the removal of the alloy from the bath.

2. The method of loosening porous oxide coating from the surface of an alloy which comprises lightly etching the surface directly beneath said coating by exposing it to the actionof an etching solution and completely severin the physical bond between the alloy and said tenacious coating by exposing the surface of said alloy immediately adjacent said coating to the action of a solution containing approximately 25 gms. of copper nitrate, and approximately 200 c. c.s of nitric acid and appreciable phosphoric acid per liter thereby causing the iron on the surface of the alloy immediately adjacent the porous coatin to replace the copper ions in the latter solution and to be replaced thereby on the surface of said alloy.

3. The method of loosening tenacious oxide coating from the surface of a ferrous base alloy article and the like which comprises activating the surface of the article in a solution of FeCla and HCl to effect a preliminary loosening of the coating by etching and then immersing the article in a bath containing approximately 25 gms. of copper nitrate and approximately 200 c. c.s of of HNO: per liter to deposit flufly copper on the surface beneath said loosened coating whereby said coating is subjected to a mild blasting force, said bath also serving to redissolve said deposited copper. g

EDWIN R. BOWERMAN, J a.

' REFERENCES orrnn The following references are of record in the file of this patent:

UN IT ED STATES PATENTS Name Date 1,415,466 Pechkranz May 9, 1922 1,526,644 Pinney Feb. 17, 1925 1,773,135 Flanzer Aug. 19, 1930 1,776,603 Schulte 'Sept. 23, 1930 1,837,118 Elder Dec. 15, 1931 2,039,328 Lukens May 5, 1936 2,295,204 Dockray Sept. 8, 1942 2,326,707 Thomas et al. Aug. 10, 1948 2,359,095 Elder Sept. 26, 19

' FOREIGN PATENTS Number Country Date 412,841 Great Britain July 5, 1986 488,656 Great Britain..- July 11, 1988 

